Network Working Group T. Williams
Request for Comments: 199 SDC
NIC: 7151 15 July 1971
SUGGESTIONS FOR A NETWORK DATA-TABLET GRAPHICS PROTOCOL
Disclaimer
The work reported herein was supported by the Advanced Research
Projects Agency of the Department of Defense under Contract DAHC15-
67-C-0149, ARPA Order No. 1327, Amendment No. 3, Program Code No.
1D30, and 1P10.
The views and conclusions contained in this document are those of the
author and should not be interpreted as necessarily representing the
official policies, either expressed or implied, of the Advanced
Research Projects Agency or the U. S. Government.
INTRODUCTION
The purpose of this document is to add SDC's comments to the
discussion of a protocol for network graphics within the ARPA Network
community. In general, we are concerned with the development of the
graphics protocol in two areas: non-interactive graphics and data-
tablet graphics, as opposed to fully interactive graphics. By non-
interactive graphics we mean situations in which there is little or
no requirement for interaction with displays. Such displays are
used, for instance, in data retrieval systems using graphics to
display retrieved information in the form of charts, X-Y graphs,
histograms, scatter plots, tabular displays, etc. In these systems,
each interaction with the system produces an entirely new display.
The displays themselves have little, if any, structure. There is no
necessity to interact with the picture itself other than, perhaps, by
the use of light buttons. It is important that non-interactive
graphics be simple to implement and use on the network. Therefore,
we suggest that the graphics protocol design be based upon non-
interactive graphics systems and that capabilities needed for
interactive graphics be added as a super-set. This will ensure that
the protocol complexities associated with interactive graphics do not
impose problems for the user of non-interactive graphics, as they
would if a non-interactive subset were developed from a protocol
based initially on interactive graphics. The section of Request for
Comment (RFC) 177 describing actual display instructions contains a
good basis for the development of a non-interactive graphics
protocol. With it as a starting point, a protocol for the generation
of a picture can be developed, and the organizational and structural
information useful for interactive graphics can be developed later.
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RFC 199 SUGGESTIONS FOR A NETWORK DATA-TABLET 15 July 1971
DATA-TABLET GRAPHIC INPUTS
Our primary topic of concern is data-tablet graphics. Though there
are a variety of data-tablet implementation, their functional
characteristics are similar enough that they can be treated as a
single class.
Data-tablet input consists of a triple of information--X, Y, Z--where
X is the distance along the abscissa, Y is the distance along the
ordinate (the two quantities are usually measured to a precision of 1
in 1024), and Z is the distance above the writing plane. There are a
variety of encodings for Z, from a simple binary quantity, on or off,
to three or more values giving various distances, from on the surface
to several inches above; for our purposes here, we will consider Z as
a binary entity.
Input timing may also vary, depending on the tablet implementation
and installation interface. Timing varies from a single shot, where
only one coordinate point is input for each new time that Z indicates
that the stylus is on the writing surface; to asynchronous, where the
tablet input is sampled on demand from the driving program or
interface logic when certain conditions are met, such as that the pen
has moved a certain amount from the previous sample or that the
program is ready for another data sample after a variable amount of
processing; to clocked synchronous, where a timing pulse provides the
sampling demand. Clock rates vary from a few (one or two) samples
per second to nearly 5000 samples per second. Some clocks are fixed,
while others are controlled either by program or external switches.
Relative to the amount of picture information contained in the data
stream, in general, the data-tablet input is far more voluminous than
a similar computer generated image. Additionally, the data-tablet
input stream contains temporal information that, in certain cases, is
vital to the proper processing of the input. Therefore, ways must be
found to implement a data-tablet graphics protocol that is flexible
enough to accommodate a broad spectrum of data volume and that is
compatible with the protocol for non-interactive display images.
PROPOSED DATA-TABLET INPUT PROTOCOLS
Data tablet input can consist of anything from a single point (as
would occur when something was being pointed at) to literally
thousands of bytes representing a hand-drawn rendering of a picture
or a line of text. In many instances, the raw data-tablet input is
preprocessed before it is passed to the principal processing program.
This preprocessing can consist of such things as a variety of
smoothing algorithms, filtering for thinning and or redundancy
removal, detection of certain operator actions such as uniquely
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RFC 199 SUGGESTIONS FOR A NETWORK DATA-TABLET 15 July 1971
marking each occurrence of placing the pen on the writing surface and
raising it, and possibly other, more exotic processes such as corner
detection, fitting straight-line segments, and the like. Most of
these latter processes will not be considered for inclusion in the
protocol, since they are usually unique to a particular investigator
and his research.
Therefore, a data-tablet graphic protocol should permit the sender to
specify, and the receiver to discriminate among, at least four types
of data-tablet input:
1) Single-shot data
2) Unpreprocessed (raw) asynchronous data
3) Unpreprocessed (raw) synchronous data
4) Preprocessed data
We will define formats for the first three, then discuss the fourth
in some detail before defining its format.
To reduce the number of bits transmitted, data-tablet information
should be transmitted in incremental form: a first point, followed
by the difference between each point and its predecessor. To
eliminate the trailing zeros that may be required for compatibility
with the standard network graphics screen, we have included provision
for a scale factor by which all increments should be multiplied
before use.
Single-Shot Data Input Format:
Byte 0: Data tablet input op code
Byte 1: Type, 0 = single shot
Byte 2-3: X - Coordinate
Byte 4-5: Y - Coordinate
8 8 16 16
+----------+----------+---------------+---------------+
| Op code | 0 | X - coord. | Y coord. |
+----------+----------+---------------+---------------+
0 1 2 3 4 5
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RFC 199 SUGGESTIONS FOR A NETWORK DATA-TABLET 15 July 1971
PREPROCESSED-DATA INPUT FORMAT
There are a variety of processes that can be applied to raw tablet
data before it is transmitted to the requesting program. For
instance, when the tablet is "noisy" or jittery, various smoothing
algorithms may be applied. The most common of these is some form of
weighted, clumped or moving average. At SDC, we have settled on an
8-point moving average when smoothing is desirable. Another fairly
common form of preprocessing is "thinning" or filtering to remove
unnecessary or redundant data points. Depending on the end use of
the data, filter "windows" can have a variety of geometries,
including square, rectangular, diamond, and circular, and the filter
may be single or double windowed. SDC currently uses a single square
window filter on all tablet input. The window size is a variable and
may be set to zero, thus, eliminating the filter.
Logically, the filter may be described as:
Take (X,Y) if |Xp - X| >= w or |Yp - Y| >= w is true
where: (X,Y) is the current data point, (Xp,Yp) is the previously
accepted data point that either passed the filter or was the first
point of the stroke, and w is the window size.
Pictorially, this can be represented as:
---- +-------------+-----
^ | | ^
| | | |
| | W
2W | Xp, Yp __|__v__
| |
| | |
v | |
-----+-------------+
| <-- 2W --> |
Any point inside the square will be rejected, any point on the
boundary or beyond is accepted and becomes (Xp,Yp). In addition to
smoothing and filtering, we have found it necessary that our
character recognition algorithms be able to estimate the velocity
along the path of the stroke. Therefore in addition to saving the X,
Y coordinates that pass the filter (smoothing, if done, precedes
filtering and is done on the raw points), we count and store the
number of rejected points between the saved ones. Since the data-
tablet input is synchronous, the count times the sampling rate
divided into the distance between adjacent points is a sufficient
approximation for our purposes. Our character-generator also
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RFC 199 SUGGESTIONS FOR A NETWORK DATA-TABLET 15 July 1971
requires the rectangle surrounding a stroke (defined by the minimum
and maximum values of X and Y in the stroke); this information is
very easy to generate during preprocessing.
Assuming that other Network nodes wanted to use SDC's tablet graphic
software--the character recognizer in particular--we would have to
know what, if any, preprocessing was done to the input data before it
was sent. Our suggested format for this from of tablet data, then,
is:
Byte 0: Data tablet op code
Byte 1: Type, 3 = preprocessed
Byte 2: Flags
Byte 3: Scale of deltas
Byte 4: Sampling rate if synchronous (as indicated by flag)
Byte 5: Window Size
Byte 6-7: Number of Points
Byte 8-9: 1st X-coordinate
Byte 10-11: 1st Y-coordinate
Byte 12-13: Minimum value of X in the stroke
Byte 14-15: Minimum value of Y in the stroke
Byte 16-17: Minimum value of X in the stroke
Byte 18-19: Minimum value of Y in the stroke
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RFC 199 SUGGESTIONS FOR A NETWORK DATA-TABLET 15 July 19718 8 8 8 8 8 16 16 16 16 16 16 16
--+-+-----+-----+-----+------+------+----+----+-----+-----+-----+-----+
Op|3|Flags|Scale|Rate |Window| # | X0 | Y0 |Xmin |Ymin |Xmax |Ymax |
| | | | | Size |points| | | | | | |
--+-+-----+-----+-----+------+------+----+----+-----+-----+-----+-----+
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
8 8 8 8 8
-----+-----+-----+ +-----+-----+
delta|delta|delta|...|delta|delta|
X1 |Y1 | X2 | |Xn |Yn |
-----+-----+-----+ +-----+-----+
20 21 22 2n+20 2n+21
Counts deleted
The flags in this format not only indicated whether or not the data
is synchronous and whether the counts are present, but may also be
used to indicate whether or not the data was smoothed and the type of
filtering.
CHARACTER SETS AND CHARACTER GENERATION
Our work in character recognition impacts the proposed protocols in
one other area, that of character sets and character generation.
Figure 1 shows the displayable characters presently available. We
have planned extensions that will bring the set to 192 characters.
The availability and use of our and others' extended character sets
must be provided for in the protocol.
The character-set problem, though, is the easy one. We have found
that when dealing with hand-printed input, the computer-generated
output must be flexible enough to retain the geometry of the user's
input--at least temporarily. This requires that we be able to
generate characters in a large variety of sizes, with variable aspect
ratios (independently specified sizes for X and Y). Since this is
not an available hardware function, all of our characters are program
generated. We currently specify character size and ratios in terms
of X and Y multipliers applied to a character prototype. The
character prototype is constructed on a 5" x 7" grid (extended, if
necessary to handle the long tails on p's, q's, etc.), where the
grid-line spacing is 2^-10 times the screen size. The important
point is that network transmission must be capable of specifying
those types of characteristics when needed.
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RFC 199 SUGGESTIONS FOR A NETWORK DATA-TABLET 15 July 1971
We propose, then, that a message format that specifies:
o Character code
o Character position
o Character height and width
As an inside, we would rather that the character origin be the left-
hand baseline point rather than the center--primarily because the
center is ill-defined unless the character space is specified to
include vertical extensions in both directions but also because it is
difficult to take advantage of variable spacing to justify characters
that are of unequal width (an aesthetic consideration of relevance in
some displays).
Figure 1: SDC EXTENDED CHARACTER SET (see PDF file)
Endnote
Subscript notation is inline. See the PDF file for a complete view
of this document.
[This RFC was put into machine readable form for entry]
[into the online RFC archives by Lorrie Shiota, 10/01]
Williams [Page 10]